i simulated the circuits in an mobile app. below is another circuit with negative feedback
-I understand how the second circuit works(-ve feedback) but the problem is with the first circuit
-Here is how the -ve feedback works(i think):
We apply 1 volt to inverting input the output keeps falling(since the input is given to inverting input) from 0V and as it approaches -1V the inverting input becomes 0V(approx)and becomes virtual ground.now the system is stable
-But in positive feedback circuit (the first one) the input is positive and so is the output when we feed it back the input only becomes more high and must take the output to +ve saturation but as you can see the simulator is not in agreement with it
-Is it the simulator fault or is my explanation?
Best Answer
You've got the explanation right, it's just that you don't understand the simulator. It has found a solution and presented it. What it has not done is a stability analysis, and in this case the circuit is not stable. The simulator has not varied the current, but rather found a set of values that "work" for a single, exact set of values.
Try this: replace your 1 mA current source with an AC current source. Let's say +/- 10 uA with a 1 mA DC offset. Perform a time-domain (transient) analysis on the circuit and watch what happens in each circuit as soon as the current varies from exactly 1 mA.